Sulfur-containing fuels, such as coal, coke, oven gas and fuel oil, typically are used to fire boilers for producing steam to generate electricity, for heating purposes or for processing purposes. The fuel is combined with air, in excess of the stoichiometric amount required for combustion, and ignited at a series of burners in an enclosed combustion chamber to generate combustion products consisting primarily of hot gases, but also containing particulates, such as fly ash. Heat is extracted from the hot gases in a conventional manner, and is used, for example, to heat water and produce steam. The hot gases flow in a downstream direction from the boiler, and eventually are exhausted to the atmosphere through a stack. After completion of the steam-producing function, residual heat remaining in the hot combustion gases can be used to preheat combustion air.
The hot gases from the combustion reaction include both solid and gaseous pollutants. Solid particulate pollutants usually are removed in an electrostatic precipitator or a bag house, or a combination thereof. Gaseous pollutants generally include the oxides of nitrogen (NO.sub.x) and sulfur dioxide (SO.sub.2). Relatively recently, the oxides of nitrogen in the combustion gases have been reduced by improved combustion techniques for oil and gas-fired boilers, and by improvements in burner design for coal-fired boilers. However, the sulfur dioxide content of the combustion gases has remained unacceptably high.
A presence of sulfur dioxide in combustion gases is especially undesirable because, sulfur dioxide escaping into the atmosphere is a source of air pollution and acid rain. Accordingly, public utilities and other large consumers of fuel, and especially fossil fuels like coal, have been forced to use a low sulfur fuel and to sufficiently remove the sulfur dioxide from the combustion gases in order to comply with government-imposed emissions standards.
The need to reduce the amount of sulfur dioxide in combustion gases poses an economic challenge to large consumers of fossil fuels. The large fuel consumers not only must use the more expensive low sulfur fossil fuels, but also must expend large sums of capital for equipment, such as scrubbers, to remove sulfur dioxide from the combustion gases. The large fuel consumers also are facing further capital expenditures as government regulators continue to reduce the legal limit of sulfur dioxide that can be introduced into the atmosphere.
Accordingly, investigators have continually sought improved methods and apparatus to effectively reduce the amount of sulfur dioxide in combustion gases emitted into the atmosphere. Such improved methods and apparatus would allow a fossil fuel consumer to meet increasingly strict government-imposed emission standards, and if sufficiently effacious, also would allow the consumer to use the less expensive high sulfur fossil fuels.
One method of reducing the amount of sulfur dioxide in combustion gases, i.e., flue gases, is known as the dry sorbent injection method. A sorbent is a compound capable of interacting with sulfur dioxide in the flue gases, in the presence of oxygen, to produce a comparatively harmless solid compound that can be separated from the flue gases and removed from the combustion chamber with conventional particulate removal apparatus. Examples of sorbents previously employed in the dry sorbent injection method to remove sulfur dioxide from flue gases include the carbonates or the hydroxides of calcium, such as particles of limestone, i.e., calcium carbonate.
In a dry sorbent injection method utilizing limestone, the sulfur dioxide in the flue gases is converted to calcium sulfate, an innocuous solid compound that can be used as a construction material or that can be buried in a land fill without adversely effecting the environment. Initially, after introduction into the combustion chamber, the particles of limestone (CaCO.sub.3) are calcined into lime (CaO) by the heat from the combustion reaction. The lime then reacts with sulfur dioxide, in the presence of oxygen (from the excess air in the combustion chamber), to produce calcium sulfate (CaSO.sub.4).
Others have attempted, at least on a test basis, to inject limestone particles into a combustion chamber having burners that produce a low percentage of oxides of nitrogen, i.e., low NO.sub.x burners. In these attempts, limestone was introduced into the combustion chamber either through the fuel nozzles, through the inlets for introducing the secondary air (located closely adjacent the fuel nozzle), through tertiary air inlets closely surrounding the inlets for the secondary air, or through separate limestone-injecting inlets spaced relatively far downstream from the inlets for the fuel and the combustion air. In the first three instances, the limestone was premixed with the fuel and/or the combustion air entering the combustion chamber at the secondary and tertiary air inlets.
Each of these limestone injection techniques possessed disadvantages. Injection either through the fuel nozzles, or through secondary air inlets immediately adjacent the fuel nozzle, or through tertiary air inlets closely surrounding the secondary air inlets subjects the limestone particles to relatively high temperatures for relatively long periods of time. This can sinter the resulting lime particles, thereby reducing the surface area of the particles and their ability to interact with sulfur dioxide, and in turn decrease sulfur dioxide removal. Introducing the limestone particles relatively far downstream from the fuel nozzles and combustion air inlets decreases sulfur dioxide removal because the temperature at the point of introduction is too low and/or decreases too rapidly to efficiently calcine the limestone to lime. Introducing lime directly into the combustion chamber does not effectively reduce the amount of sulfur dioxide in flue gases.
Premixing the limestone particles with the combustion air also causes erosion and pluggage problems in the conduits that transport combustion air, and substantially reduces the ability to divide the limestone particles into substreams for introduction into the combustion chamber by a plurality of individual air outlets. These problems arise from the high velocity at which the combustion air flows through the transporting conduits, e.g., about 2,500 to about 5,000 ft/min. (about 762 to about 1524 m/min.), and the fact that the limestone particles are carried in dilute phase transport, i.e., a high air to limestone particle ratio.
If the velocity of the transporting combustion air is reduced to decrease the erosion and pluggage problems, the volume of the transporting air has to be increased in order to carry the limestone in dilute phase transport at the slower speeds. This can result in more combustion air at a given outlet, or series of outlets, than desired from the standpoint of efficient combustion or other considerations. Moreover, lowering the velocity at which the combustion air is introduced into the combustion chamber reduces the turbulence and mixing action provided by the combustion air, and such a reduction in turbulence is undesirable. A minimum combustion air velocity is necessary for the combustion air to properly distribute the limestone particles carried by the combustion air within the combustion chamber.
The dry sorbent injection method and attendant apparatus for reducing the amount of sulfur dioxide in a combustion gas are fully described in Landreth et al. U.S. Pat. Nos. 5,027,723 and 5,048,431, hereby incorporated by reference. In general, the dry sorbent injection method, as described above, includes introducing a solid sorbent into a combustion chamber, such as a boiler, in a zone of the combustion chamber above the fuel and flame area. This zone is of a sufficient temperature to calcine the solid sorbent, and allow the calcined sorbent to interact with sulfur dioxide present in the combustion gases and form a solid inorganic sulfate. For example, a sorbent, like hydrated lime (i.e., Ca(OH).sub.2) is introduced into a boiler at a zone above the fuel and flame area, wherein the hydrated lime is calcined to lime (i.e., CaO). In the presence of oxygen, the lime then interacts with sulfur dioxide to form calcium sulfate. The solid calcium sulfate separates from the flue gases and is collected and removed from the boiler by standard methods of removing ash from a boiler.
Under optimum conditions, the dry sorbent injection method disclosed by Landreth et al. reduces the amount of sulfur dioxide in combustion gases by approximately 50%. At this rate, the amount of sulfur dioxide is reduced to a sufficient extent to meet present-day emissions standards, even if a fossil fuel containing a medium amount of sulfur, e.g., about 3% by weight, is consumed. However, current emissions standards would not be met if a high sulfur fossil fuel is consumed, and a fuel consumer would not meet the more strict emissions standards scheduled to go into effect in 1995 and 2000, even if a low sulfur fossil fuel was used.
Therefore, it would be desirable to provide a method of reducing the amount of sulfur dioxide in combustion gases generated in a combustion chamber that burns fossil fuel such that: (a) stringent emissions standards can be met without the need for additional capital expenditures for sulfur dioxide-removal equipment and/or (b) medium or high sulfur-containing fuels can be used without violating present or future emissions standards. The present invention is directed to such an improved method, wherein improved sorbent composition is introduced into the combustion chamber using the dry sorbent injection method.